PICVib: an accurate, fast and simple procedure to investigate selected vibrational modes at high theoretical levels and evaluate Raman intensities

Abstract

Raman intensities and depolarization ratios are important properties to assign the vibrational spectrum and thus to characterize substances and materials. Practical and reliable computational methods are relevant to guide and aid the assignment and interpretation of Raman spectra. The procedure for investigating categories of vibrations (PICVib) approach is generalized for the Raman intensity calculations, and it is shown to be successful and to preserve all interesting features of the procedure such as easiness of implementation, parallelization, flexibility and treatment of large systems at high theoretical levels. The PICVib method involves calculations of the polarizability tensors at the displaced structures to provide the Raman intensities of selected normal modes, in addition to their vibrational wavenumbers and infrared intensities. It was validated for very diverse molecular systems: XH3 (D3h), YH4 (D4h), conformers of 1,3,5-trinitro-1,3,5-triazacyclohexane, SN2 and E2 product complexes, fullerene C60, lanthanide ion aquo complexes and hydrogen-bonded complexes (water–water, water–methanol and methanol–methanol) including the guanine-cytosine pair. This assessment shows an excellent overall performance of the PICVib method for calculating Raman intensities of localized normal modes and even mixed vibrations. For vibrations involving intermolecular complexes, the choice of the lower level method (e.g. density functionals and semiempiricals) is crucial. This makes the PICVib procedure practically complete for vibrational analysis of molecular systems and should now be generalized for extended and periodic systems. Copyright © 2016 John Wiley & Sons, Ltd.